[Bug debug/107414] dwarf 5 C macro support

["simon.marchi at polymtl dot ca" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107414

Simon Marchi <simon.marchi at polymtl dot ca> changed:

           What    |Removed                     |Added
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                 CC|                            |simon.marchi at polymtl dot ca

--- Comment #5 from Simon Marchi <simon.marchi at polymtl dot ca> ---
The original reproducer Ulrich gave indeed seems like what was fixed in GDB
master recently.  Macros in the main source file didn't work with DWARF 5. 
That works fine now.

But he pointed me to a real-life case in gawk, where it didn't work:

https://sourceware.org/bugzilla/show_bug.cgi?id=29725#c2

That seems to highlight a new problem, that is the use of macros combined with
#line directives.  I don't think that the debug info is sufficient for GDB to
figure this out.

For convenience, here's a copy of what I posted on the GDB bug.

---

With a simple case, it works as expected:

$ cat test.c
#define FOO 2
int main() { return FOO; }
$ gcc test.c -g3 -O0
$ ./gdb --data-directory=data-directory -nx -q -iex "set debuginfod enabled
off" ./a.out -ex start -ex "info macro FOO" -batch
...
Defined at /home/simark/build/binutils-gdb/gdb/test.c:1
#define FOO 2

But then if you add a line control directive, like you have for a .c file
generated from a .y file:
$ cat test.c
#define FOO 2
#line 17 "potato.c"
int main() { return FOO; }
$ gcc test.c -g3 -O0
$ ./gdb --data-directory=data-directory -nx -q -iex "set debuginfod enabled
off" ./a.out -ex start -ex "info macro FOO" -batch
...
The symbol `FOO' has no definition as a C/C++ preprocessor macro
at /home/simark/build/binutils-gdb/gdb/test.c:-1

Looking at the macro debug info (my annotations after the #s):

$ readelf --debug-dump=macro a.out

 DW_MACRO_import - offset : 0x20 # built-in macros
 DW_MACRO_start_file - lineno: 0 filenum: 2 # test.c
 DW_MACRO_start_file - lineno: 0 filenum: 3 # stdc-predef.h
 DW_MACRO_import - offset : 0x900 # STDC macros
 DW_MACRO_end_file # end of stdc-predef.h
 DW_MACRO_define_strp - lineno : 1 macro : FOO 2
    <--- here
 DW_MACRO_end_file # end of test.c

The line table tells GDB that the current PC is in potato.c, line 17:

CU: ./test.c:
File name                            Line number    Starting address    View   
Stmt
potato.c                                      17              0x1119           
   x
potato.c                                      17              0x111d           
   x
potato.c                                      17              0x1122           
   x
potato.c                                       -              0x1124

But GDB has no way to figure out where this is in the macro inclusion tree, as
potato.c is never represented in there.  Ideally, GDB would figure out that we
are where I maked `here`, above.

Looking at the DWARF opcodes for macro, I don't really see a way to describe
the #line stuff.  There's only DW_MACRO_start_file, which represents the
inclusion of a complete file.